A typical oil or gas production well comprises lengths of steel production tubing through which the product is conveyed from the subterranean reservoirs(s) to the wellhead, surrounded by one or more generally coaxial tubular well casings. The outer casing is generally a concrete lining to the earth formations surrounding the wellbore, with one or more intermediate casings (particularly in the case of sub-sea wells) therebetween. Each circumjacent pair of tubular structures define a generally annular space between them, and these annuli are conventionally identified alphabetically as the A-annulus, B-annulus etc working outwards from the production tubing to the outer casing.
For the safe and efficient operation of such a well there is a need to monitor various environmental parameters down-hole and the production tubing is typically equipped on its exterior with various instrumentalities and cabling for powering, controlling and transmitting data from numerous sensors and tools associated with that tubing and the A-annulus. Monitoring of certain conditions within the B-annulus is also desirable and in particular it is important to monitor the pressure within that annulus to provide a warning of any leakage down-hole which may give rise to hazardous conditions. The temperature within that annulus may additionally or alternatively need to be monitored. Regulatory conditions prohibit penetration of the wellhead for measurement of B-annulus parameters, however, which means that one or more sensors have to be located in that annulus at a suitable down-hole position or positions. An electrical penetrator through the wall of the respective intermediate casing or other means can be used to communicate with such a sensor from the A-annulus. It is undesirable, however, to run a cable from topside to this position along the interior surface of the intermediate casing as the production tubing may need to be run into and out of the well on various occasions during its lifetime and may cause damage by collision with such cabling during the process. Furthermore since the production tubing itself will in any event usually be equipped with suitable cabling and gauge controllers or the like it should be more cost efficient to employ the same for communication also with the B-annulus pressure sensor(s), subject only to bridging the gap in communication across the width of the A-annulus. In the latter respect a hard wired connection is undesirable due to the likely need to withdraw the production tubing on occasion and the consequent difficulties of making and breaking the required connection, the possible presence of debris in that annulus, and the general need to avoid obstruction.
With the foregoing in mind, in one aspect the invention resides in apparatus for the wireless transmission of data across a generally annular space between a length of production tubing and a surrounding casing in a petrochemical well, comprising a pair of coils of different diameters adapted to be inductively coupled in a generally coaxial disposition, a first of which is adapted to be located on the production tubing generally coaxially therewith and the second of which is adapted to be located on the casing generally coaxially therewith.
In use the first coil is preferably located on the exterior of the production tubing and the second coil is preferably located on the interior of the casing.
Instrumentation may be associated with the second coil which in use is powered by inductive coupling from the first coil.
For the purposes of data transmission there may be means for applying a time-varying current to the first coil, means for modulating the load on the second coil, and means for detecting consequent amplitude modulation of the voltage across the first coil.
In the case of B-annulus pressure and/or temperature monitoring as described above, the first said coil can be associated with a reader on the exterior of the production tubing while the second said coil can be associated with a transponder on the interior of the intermediate casing, the latter receiving data from a B-annulus sensor or sensors which can be transmitted wirelessly from the transponder to the reader by virtue of the inductive coupling of the coils. By the same token, both the transponder and sensor(s) are preferably powered by the current induced in the second coil by the first, meaning that these can be “passive” devices requiring no other power source. It is also, however, within the scope of the invention for these to be “active” devices with a battery or other power source in other embodiments.
In another aspect the invention accordingly resides in a petrochemical well installation comprising: a length of production tubing; a first casing surrounding said tubing and defining therewith a first generally annular space (“A-annulus”); a second casing surrounding said first casing and defining therewith a second generally annular space (“B-annulus”); a sensor for sensing a parameter within the B-annulus; a transponder located on the interior of said first casing with a coil generally coaxial with said first casing; means providing signal communication between said sensor and transponder; and a reader located on the exterior of said tubing with a coil generally coaxial with said tubing; whereby in use said coils are inductively coupled across the A-annulus for the transmission of data from said sensor to said reader via said transponder.